A supporting structure within a furnace chamber where temperatures may be in the order of 2000.degree. F. must be compact in order to leave sufficient combustion space in the chamber, must be strong enough support heavy metal work pieces being treated in the furnace, and must be protected against injury by the high temperature within the furnace while at the same time not seriously interfering with the efficiency and maximum temperature of a furnace. It must also be strong enough to withstand the stresses and heavy vibration set up by the movement of the heavy workpieces within the heating chamber.
A combination of small size with high strength dictates the use of metals in the supports and the necessity for cooling the metal dictates the use of hollow metal pipes through which cooling water is circulated.
If water cooled pipes with bare outer surfaces are used, however, the absorption of heat through the pipe metal to the cooling water is so great that more fuel is wasted in heating the cooling water than in heating the work pieces. For example, an under fired furnace having bare metal supports in its lower zone requires about three times as much fuel to heat the lower zone of the furnace as the upper zone of the furnace, and no matter how much fuel is supplied to the lower zone it still remains distinctly cooler than the upper zone.
In an endeavour to correct this condition it has been proposed to apply refractory insulating material directly to the water cooled support pipes. However, this expedient has not proved very satisfactory because the refractory material tends to crack after some use, principally because of the difference in thermal expansion of the inner and the outer layers of the refractory materials and because of the difference in the thermal expansion characteristics of the refractory materials and the metal pipe which it encloses. The movement of the supporting structure due to the movement of the heavy work pieces within the heating chamber also contributes significantly to the break up and loss of the refractory material particularly after cracking has occurred.
It is therefore advantageous that the insulation be so constructed that if cracking of the refractory material does occur, movement of the piping does not result in the complete break-up and loss of the refractory material. It has been proposed that this be accomplished by embedding in the refractory material a reticulated metal structure such as a wire fabric comprising interlocking coils of wire extending parallel to each other and to the pipe axis to act as a reinforcement and support for the refractory material which, should damage to the refractory material occur, will hold the refractory material together and in situ round the piping thus preserving the heat insulation.
To facilitate the insulation of such an insulation on piping the insulation has conveniently been made in two or more preformed sections which are secured together circumferentially around the piping. For example, a two-section insulation has included a preformed semi-cylindrical refractory element having wire fabric embedded therein with portions of the wire protruding from the refractory material such that when the section is placed against the pipe the protruding wire portions are situated adjacent opposite sides of the pipe. The protruding portions of the wire are then welded to the pipe.
While the above form of insulation is capable of giving very satisfactory results, the need for welding the protruding portions of the wire to the pipe is time-consuming and costly.